- to 
-phase transformation may not fully
explain the
topography of the 400-km discontinuity, i.e., dynamical and chemical
effects may be
important near
the top of the transition zone.
Department of Earth and Planetary Sciences
Harvard University
Cambridge, MA 02138
Email:
gu@eps.harvard.edu
poster/oral: poster
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Until now, modeling of three-dimensional (3-D) velocity variations in
the mantle
and topography of the transition zone discontinuities have been
considered separately. Velocity models were obtained assuming that
the radii of the discontinuities are constant. Then, the travel time
data sensitive to the topography, such as the SS precursors,
were corrected for the effect of 3-D structure and inverted
for depth variations of a discontinuity. Such a procedure
is unsatisfactory, as it may introduce artifacts that could
significantly
affect the topography results; the opposite trade-off is less likely
to introduce conceptually important change in the velocity
distribution but should also be considered.
In this study we bring together the same set of S-velocity
sensitive data as used by Gu et al. (2002) and combine it with
a large set
of differential travel times
of SS-SdS ( d for the 400- or the 670-km discontinuity) as
well as
direct
measurements of S400S-S670S (Gu and Dziewonski, 2002). After
applying
corrections
for the crust, topography and bathymetry, we formulate the inverse
problem
in terms of the volumetric (3-D) and topographic (2-D) perturbations
for both the 400-km and 670-km discontinuities.
The best fit model of the joint inversion significantly improves the
variance reduction of SS-S400S and SS-S670S data sets.
The velocity distribution in the resulting model, TOPOS362D1,
is very similar to that in model S362D1 (correlation
coefficient above 0.8 throughout the mantle), but important changes
have been made to the topography of the 400- and 670-km
discontinuities
with respect to those obtained earlier assuming a particular velocity
model.
The amplitude of the 400-km discontinuity decreased significantly
compared
with Flanagan and Shearer (1998) and Gu et al. (1998); in
TOPOS362D1 its
maximum variation does not exceed 12 km. In particular, the strong
degree-1
component before the joint inversion has decreased, such that the
correlation
between the velocities
above the discontinuity and the shape of the discontinuity itself has
substantially diminished. Spatially, this result means a significant
diminution in the amplitude of the earlier reported depression of the
400-km
discontinuity under the Pacific (e.g., Flanagan and Shearer, 1998).
The power
spectrum of the
topography of the 670-km discontinuity
has been enriched in long wavelength component, especially degree 2.
The range of depth variations is +/-18~km and its shape correlates
well
with the radially averaged velocity perturbations in the transition
zone. The
best correlation is observed near region of significant current
subduction, e.g.,
in the
western Pacific and
South America, which suggests a likely presence of laterally spread
and/or
accumulated
oceanic lithosphere in the transition zone. At wavelengths
greater than
1000 km, there is little correlation between the depth perturbations
of the 400- and 670-km discontinuities. The depth variation
of 400-km discontinuity does not appear to be strongly influenced by
thermal
structures potentially associated with subduction and plumes. This
implies that
thermal influence
on the olivine - to -phase transformation may not fully
explain the
topography of the 400-km discontinuity, i.e., dynamical and chemical
effects may be
important near
the top of the transition zone.
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